1. Field of the Invention
The present invention relates to a synchronous motor control system for controlling a synchronous motor driven by a power source of n-phase alternating currents, which draw sine waves as their basic waveform, and also to a method of controlling such a synchronous motor. More specifically the present invention pertains to a synchronous motor control system for arbitrarily controlling the characteristics of such a synchronous motor as well as to a method of controlling the same.
The present invention also relates to a synchronous motor control system for controlling each phase current of a synchronous motor, which drives and rotates a rotor by an interaction of a magnetic field produced by field coils through which a sine-wave multi-phase alternating current flows, with a magnetic field produced by permanent magnets mounted on the rotor, and also to a method of controlling such a synchronous motor. More specifically, the present invention pertains to a technique for improving mechanical outputs of such a synchronous motor.
2. Description of the Related Art
Synchronous motors giving stable rotating characteristics at a synchronizing speed have been utilized in a variety of fields. The advance of semiconductor technology enables the frequency of a power source to be easily varied and realizes a wide range of control for the revolving speed of the synchronous motor. This further expands the applicable field of the synchronous motors. The synchronous motors have recently been applied to the high-torque requirements, for example, as the power source of electric vehicles.
Known improvement in a conventional synchronous motor control system includes varying the revolving speed of the synchronous motor over a wide range and smoothing the rotation of the synchronous motor to a possible extent.
In synchronous motors using n-phase alternating currents as a power source, the phase of producing the primary magnetic flux is shifted at every 180/n degrees of electrical angle. This undesirably causes torque ripples. A variety of techniques have been proposed in order to reduce the fluctuated rotation due to the torque ripples. By way of example, a proposed control system decreases the phase current at an electrical angle corresponding to a maximum momentary torque and reduces the maximum torque in the cycle of torque ripples, thereby reducing the torque ripples. The proposed techniques also include lowering the general torque command value, making parts of the armature windings of the adjacent phases to be overlapped with each another to depress the variation in magnetic flux accompanied with the phase shift, and adjusting the gap length between the rotor and the stator. Another proposed structure obliquely cuts specific ends of permanent magnets along the rotational axis to reduce a fluctuation in torque.
These proposed techniques reduce torque ripples of a synchronous motor where lowering the mechanical output. In the technique of decreasing the input electric current in the range of electrical angles corresponding to greater torques for the purpose of reducing torque ripples, the decrease in input electrical energy lowers the mechanical outputs of the synchronous motor and thereby decreases the mean output torque. Another technique of making the windings dispersed in adjacent phases of the synchronous motor undesirably decreases the primary magnetic flux, which is dominant in generation of the torque of the synchronous motor. This lowers the maximum output torque per volume of the synchronous motor.
The lowered output accompanied with the reduction of torque ripples may cause problems in some fields to which the synchronous motor is applied. For example, in the case that the synchronous motor is used as a power source of electric vehicles, the decrease in mechanical output per unit weight of the synchronous motor is undesirable. When the minimum torque is less than a predetermined level, the synchronous motor may be stepped out under the condition of large loading. The decrease in maximum mechanical output prevents the electric vehicle from running satisfactorily under the condition requiring large torques, for example, at the time of starting or ascending a slope.
Thus, these improvements attain some reduction of torque ripples, but do not control the output torque of the synchronous motor according to the rotational angle thereof.
The synchronous motor control system of the present invention is based on the novel conception, that is, to control the output torque of a synchronous motor according to the electrical angle thereof.